The effect of hydrophobic glassy organic material on the cloud condensation nuclei activity of particles with different morphologies

The effect of hydrophobic glassy organic material on the cloud condensation nuclei activity of particles with different morphologiesEffect of hydrophobic glassy organics on CCN activity of mixed particlesAnkit Tandon et al.

Particles composed of organic and inorganic components can assume
core-shell morphologies. The kinetic limitation of water uptake due to the
presence of a hydrophobic viscous outer shell may increase the critical
supersaturation required to activate such particles into cloud droplets. Here
we test this hypothesis through laboratory experiments. Results show that the
viscosity of polyethylene particles is 5×106 Pa s at
60 ∘C. Extrapolation of temperature dependent viscosity measurements
suggests that the particles are glassy at room temperature. Cloud
condensation nuclei (CCN) activity measurements demonstrate that pure
polyethylene particles are CCN inactive at diameters less than 741 nm and
2.5 % water supersaturation. Thus, polyethylene is used as proxy for
hydrophobic glassy organic material. Ammonium sulfate is used as proxy for
hygroscopic CCN active inorganic material. Mixed particles were generated
using coagulation of oppositely charged particles; charge-neutral
polyethylene–ammonium sulfate dimer particles were then isolated for online
observation. Morphology of these dimer particles was varied by heating, such
that liquefied polyethylene partially or completely engulfed the ammonium
sulfate. Critical supersaturation was measured as a function of dry particle
volume, particle morphology, and organic volume fraction. The data show that
kinetic limitations do not change the critical supersaturation of 50 nm
ammonium sulfate cores coated with polyethylene and polyethylene volume
fractions up to 97 %. Based on these results, and a synthesis of
literature data, it is suggested that mass transfer limitations by glassy
organic shells are unlikely to affect cloud droplet activation near
laboratory temperatures.

Atmospheric aerosol sometimes consists of mixed particles with inorganic and
organic components present in comparable mass fractions (Cruz and Pandis,
2000; Pöschl et al., 2010; Prenni et al., 2003). Most of the inorganic
compounds, commonly salts, are hygroscopic. The organic fraction is composed
of a large number of compounds, many of which remain unspeciated. The
contribution of the organic fraction to cloud condensation nuclei (CCN)
activity depends on their hygroscopicity. The effective hygroscopicity
parameter (Petters and Kreidenweis, 2007) of organic compounds varies between
0 and ∼ 0.3, which is based on laboratory, field, and modeling studies
(e.g., Prenni et al., 2007; Gunthe et al., 2009; Mikhailov et al., 2009; Chang
et al., 2010; Massoli et al., 2010; Cappa et al., 2011; Mei et al., 2013;
Pajunoja et al., 2015; Petters et al., 2016; Nakao, 2017). A small subset of
species constituting the organic fraction, e.g., long-chain n-alkanes with
carbon number 16 and above and their fatty acids, have very low water affinity
(Jacobson et al., 2000; Jimenez et al., 2009; Petters et al., 2016). The
organic fraction can thus be divided into hydrophobic and hydrophilic
compounds. Hydrophobic compounds do not take up water and do not contribute
to CCN activity.

Particles containing both organic and inorganic compounds may assume
different phase states and morphologies. The phase state of dry organic
matter may be crystalline, amorphous solid or semisolid, or liquid. When
formed from drying of aqueous solution droplets, the organic phase often
partially or fully encapsulates the inorganic core (e.g., Ciobanu et al.,
2009; Freedman et al., 2010; Reid et al., 2011; Krieger et al., 2012; Stewart
et al., 2015; Altaf and Freedman, 2017). Bigg (1986) first suggested that
organic compounds located at the particle surface may kinetically retard
water uptake and thus reduce the particle's ability to promote CCN activity.
This reduction in CCN activity may occur through several pathways. First, the
mass accommodation coefficient of water may be reduced (e.g., Chuang et al.,
1997; Nenes et al., 2001). Second, slow dissolution kinetics may reduce the
number of dissolved molecules in the aqueous solution (Asa-Awuku and Nenes,
2007; O'Meara et al., 2016). Finally, high viscosity may slow diffusion of
water (e.g., Zobrist et al., 2011) and thus affect hygroscopic growth under
subsaturated conditions (Mikhailov et al., 2009), especially at cold
temperature (Berkemeier et al., 2014; Petters et al., 2019). Measurements of
the diffusion coefficient of water through hydrophilic viscous matrices
(Price et al., 2014), measurements of the equilibration timescale of glassy
hydrophilic organic compounds with water at high relative humidity (RH;
Rothfuss et al., 2018), and measurements of the interplay between viscosity,
hygroscopic growth, and CCN activity (Pajunoja et al., 2015) suggest that
hydrophilic liquid or hydrophilic amorphous solid organic matrices are
unlikely to hinder droplet activation. The main reason for this behavior is
that small amounts of water uptake plasticize the organic phase, which in
turn removes diffusion limitations. Plasticization, however, does not occur
in hydrophobic organic substances, which therefore are more plausible
candidate compounds to kinetically limit water uptake and droplet activation.

Studies at subsaturated RH have been performed by several investigators.
Hämeri et al. (1992) show that coatings of
tetracosane (solid alkane), octanoic acid (liquid carboxylic acid), or
lauric acid (solid carboxylic acid) do not prevent NaCl particles from
deliquescing. Garland et al. (2005) reported that the deliquescence relative humidity for ammonium sulfate (AS)
particles coated with palmitic acid was the same as for pure AS for
particles with low organic mass fraction (∼ 20 %). For
particles with higher mass fraction (∼ 50 %) of palmitic
acid, full deliquescence required higher relative humidity. Xiong et al. (1998) experimentally examined
the effects of organic films on the kinetic growth rate of ultrafine
sulfuric acid aerosols in relation to film thickness and particle diameter.
They reported that the monolayer films of lauric and stearic acids retard the
hygroscopic growth rate of sulfuric acid aerosols. This reduction in
hygroscopic growth was 20 % for a 6-monolayer-thick (14.4 nm) coating of
lauric acid on 40–120 nm particles and RH of up to 85 %.
The slopes of hygroscopic growth factor–RH curves decrease significantly
with increasing number of monolayers and residence time. However, for films
≥14.4 nm, the growth factor–RH curve remains the same regardless of
coating thickness. Ruehl and
Wilson (2014) measured the droplet size formed at 99.9 % RH on
submicrometer particles. Ammonium sulfate particles coated with palmitic and
stearic acids did not take up water as expected suggesting an accommodation
coefficient of 10−4. Forestieri et al. (2018) report similar observations for
NaCl particles coated with
palmitic acid. Particles exceeding 80 % organic fraction had reduced
humidified droplet size at 99.88 % RH, which was attributed to kinetic
limitations. Gorkowski et al. (2017)
created core-shell structures with secondary organic materials comprising
the shell and found no diffusion limitations for water (and other
substances) at humid conditions.

Studies examining the effect of organic coatings on CCN activation also show
mixed results. Cruz and Pandis (1998) investigated the effect of organic
coatings on the CCN activation of AS particles coated with glutaric acid or
dioctyl phthalate (DOP). The CCN activation behavior of a particle having an
inorganic core with organic coating was predicted by Köhler theory and
applying the Zdanovskii, Stokes, and Robinson (ZSR) assumption that the
equilibrium water content of the mixed particle is additive. Particles with
coatings of DOP constituting at least 70 % mass of the mixed particle did
not hinder activation. In contrast, Abbatt et al. (2005) observed a complete
deactivation of AS particles that were thickly coated (>13 nm) with
stearic acid, although thinner coatings and other substances had no effect.
Nguyen et al. (2017) reported that NaCl particles coated with palmitic and
stearic acids lead to deviations from the ZSR rule consistent with kinetic
limitations to water uptake. Unsaturated acids did not show any deviation
from the ZSR rule. Forestieri et al. (2018) investigated the CCN activity of
NaCl particles coated with oleic acid and mixtures of myristic
acid ∕ oleic acid and palmitic ∕ oleic acid and found no deviation
from the ZSR rule that would be indicative of kinetic limitations.

The most commonly used materials applied in coating studies are n-alkanes
and fatty acids. These compounds are hydrophobic and CCN inactive (Petters et
al., 2016). The phase state of these molecules is assumed to be either liquid
or solid based on the bulk phase of the substance at room temperature. In
most studies the proposed mechanism responsible for the hypothesized kinetic
limitation has been a reduced mass accommodation coefficient, which hinders
the kinetic rate of droplet growth. The mass accommodation coefficient is
defined as the fraction of molecules that stick to the particle upon
collision with the droplet. Related are kinetic limitation due to diffusional
transfer through the organic material, which is controlled by the viscosity
of the coating. Viscosity ranges from 10−3 Pa s for liquids to
>1012 Pa s for glassy substances (Reid et al., 2018). According to the
Stokes–Einstein relationship the characteristic time required for water
molecule to diffuse through a 10 nm thick coating is 69, 6.9×104,
and 6.9×107 s for viscosities of
106, 109, and 1012 Pa s, respectively. Although it is known
that the Stokes–Einstein relationship may not hold for water and high
viscosity (Chenyakin et al., 2017; Price et al., 2014), increased viscosity
still slows diffusion.

The above-cited studies did not quantify the viscosity of the coating. This
study is unique in this aspect. Polyethylene (PE) is used as a proxy for the
organic material. A dimer coagulation, isolation, and coalescence (DCIC)
technique (Marsh et al., 2018; Rothfuss and Petters, 2016) is used to
generate mixed particles composed of PE ∕ PE and PE ∕ AS dimer
particles. Viscosity and CCN activity of pure PE particles is quantified.
Dimer morphology is varied from agglomerate to core-shell morphology.
Critical supersaturation (Sc) for these mixed particles is measured as a function
of dry particle volume, particle morphology, and organic volume fraction.
Atmospheric implications of the experiments are discussed.

The basic methodology for dimer particle preparation has been presented in
detail elsewhere (Marsh et al., 2018; Rothfuss and Petters, 2016). Briefly,
two monodisperse particle populations of opposite charge are generated using
differential mobility analyzers (DMAs) operated with positive and negative
polarity power supplies. The streams are merged and particles are given time
to coagulate. Coagulated dimer particles from particles carrying +1/-1 charge are charge neutral and transmit
through an electrostatic filter. The terms “monomer particle” or monomer
are used to refer to particles transmitted by a single DMA and the terms
“dimer particle” or dimer are used to refer to coagulated particles. These
particles are available for further manipulation and measurement.

Figure 1 summarizes the specific setup that was used in this study. Ammonium
sulfate particles were generated using a constant output atomizer (TSI 3076)
that was supplied with 0.1 % w∕w AS solution using a syringe pump
operated at a 40 µL min−1 flow rate. The particles were dried
using two Nafion dryers (Perma Pure PD-50T-24MSS) and charge neutralized using
a 210Po source. Monodisperse negatively charged 50 nm AS particles were
selected using the first DMA operated with sheath-to-sample flow ratios of
5 L min−1: 1 L min−1 and connected to a positive polarity
power supply (TSI-DMA1, TSI 3071). Monodisperse AS particles prepared in this
way are nearly spherical with a dynamic shape factor of 1.02–1.06, depending
on diameter and drying rate (Mikhailov et al., 2009; Zelenyuk et al., 2006).

Polyethylene particles were generated by homogeneous nucleation from vapor.
A few beads of a PE standard (Restek, Polywax-850) were heated above their
melting point (107 ∘C) in a round-bottom two-neck glass flask.
Dry particle-free N2 boiled off from a dewar was supplied through the
central neck. Nitrogen was used to prevent oxidation. A polydispersion of
particles formed from homogenous nucleation and Brownian coagulation.
Particles were size-selected using a second DMA (TSI-DMA2, TSI 3071) also
operated with sheath-to-sample flow ratios of 5 L min−1: 1 L min−1
but using a negative-voltage power supply to select positively charged
particles. The polydisperse PE particles are likely glassy at room
temperature (discussed further below) and heavily agglomerated. Therefore,
the PE particles were passed through two sintering loops. The first
sintering loop is after the initial cooling of the PE aerosol, the second
after the TSI-DMA2. Within the sintering loop the aerosol is heated to
>90∘C, which lowers the viscosity sufficiently for
the particles to relax to solid near-spherical particles.

The outflows from TSI-DMA1 and TSI-DMA2 were merged into three sequentially
arranged 0.3 L capacity coagulation chambers (27 s average residence time).
The flow was split after the chambers with 0.6 L min−1 being passed
through an electrostatic filter to remove all charged particles. The charge
neutral particles then passed through a temperature-controlled loop with
residence time 5 s. The elevated temperature liquefied the PE such that
dimer particles changed from a dumbbell morphology into to a spherical
morphology. Particles exiting the conditioner were charge equilibrated using
a 210Po radiation source. The size distribution of coagulated particles
was measured using a radial DMA (RDMA, Zhang et
al., 1995) interfaced with a condensation nuclei (CN) counter (TSI 3020) and
a streamwise thermal-gradient continuous flow CCN counter
(Roberts and Nenes, 2005) as particle
detectors. Both the CN and CCNc were operated at a flow rate of 0.3 L min−1.
The CCNc and RDMA were operated using a
sheath-to-sample flow ratio of 10:1 and 2 L min−1: 0.6 L min−1,
respectively. The RDMA was configured to measure the size distribution in
scanning mobility particle sizer (SMPS) mode (Wang and
Flagan, 1990), scanning voltage from 3000 to 100 V (145–26 nm) over 210 s.
The coagulation chamber, electrostatic filter, and RDMA are housed inside
a temperature-controlled box that can be varied between −20 and +40∘C. The temperature of the box is set to a baseline
temperature wherein dimers do not relax to a spherical shape and is referred
to as the system temperature. Relative humidity at the system temperature
was less than 3 %, much drier than conditions where ammonium sulfate
dimers were observed to relax to a spherical shape due to water uptake in
Rothfuss and Petters (2016).

Since the melting point of AS (280 ∘C; Lide and Haynes, 2009) is
much higher than that of PE (107 ∘C), the liquid PE is expected to
partially or fully engulf the AS core. The assumed morphology of a liquid
droplet on a spherical solid substrate depends on the solid ∕ liquid,
solid ∕ vapor, liquid ∕ vapor tensions, as well as the tension of the
three-phase contact line (Iwamatsu, 2016). The equilibrium morphology of this
system can be (1) core-shell morphology (complete wetting), (2) core-shell
morphology with uneven coating thickness (cap-shaped droplet), (3) partially
engulfed morphology (cap-shaped droplet), or (4) dumbbell-shaped morphology
(complete drying). Parenthetical terms correspond to those used by Iwamatsu
(2016). The line tension term becomes increasingly important at smaller
scales. Line tension drives the system toward core-shell morphology. There is
insufficient information about the AS ∕ PE system at 95 ∘C to
theoretically evaluate the equilibrium morphology. However, the
liquid ∕ vapor tension of polyethylene melts is 0.034 J m−2
(Dettre and Johnson, 1966; Owens and Wendt, 1969) and in principle a low
liquid ∕ vapor tension will favor the spreading of liquid on a
high-energy surface. Reid et al. (2011) argue that core-shell morphologies
are favored for a liquid phase surrounding a solid crystalline core, as is
the case for liquid PE spreading over crystalline AS. However, this
assumption is often invalid for particles containing hydrophobic and
hydrophilic liquid domains (Reid et al., 2011). Freedman et al. (2010) show
that ammonium sulfate particles coated with palmitic acid (produced through a
coating oven) remain in the core-shell structure. Palmitic acid and liquid PE
both have a low liquid ∕ vapor interfacial tension. Palmitic acid and PE
are both dominated by CH2 functional groups and thus the
solid ∕ liquid tension and line tension terms for these two substances
should be similar.

The particles are cooled down to system temperature after they relax to
a spherical shape in the temperature-controlled loop. Based on the above we
argue that the final morphology is a core-shell structure. However, we cannot
dismiss the possibility that particles formed a partially engulfed
morphology. That morphology may be either due to equilibrium shape or arise
during the annealing of the particle. Thus, whether the coating of annealed
PE is of uniform thickness is unknown. Scanning electron microscopy with
energy dispersive X-ray spectroscopy of PE ∕ AS dimer particles with
larger diameters was attempted. However, the analysis was inconclusive due to
insufficient resolution resulting from poor conductivity of the prepared
samples and necessary limits in maximum electron beam intensity to ensure
that the PE did not melt.

2.1 Experiment types

2.1.1 Viscosity measurement

Viscosity of pure PE particles was measured as described in previous
publications (Marsh et al., 2018; Rothfuss and Petters, 2016, 2017). For this
experiment, PE ∕ AS and PE ∕ PE dimers were prepared by feeding AS or
sintered PE particles to TSI-DMA1 and sintered PE particles to TSI-DMA2. The
temperature inside the thermal conditioner was continuously scanned from
25 ∘C (system temperature) to 95 ∘C over a period of
284 min. The mode of the size distribution was monitored by the RDMA-CPC/CCNc
instruments. The analysis of the data follows the same steps described
previously and only given in abridged form. First, the raw size distribution
is fitted to a lognormal distribution function to find the peak diameter.
Then, the particle geometry factor is determined (Rothfuss and Petters, 2016),

(1)ξ=3Duc/Dc-1DpDc+Duc/Dc-43,

where ξ is the particle geometry factor, Dp is the SMPS peak
diameter, and Duc the fully uncoalesced and Dc is the fully coalesced
dimer diameter. The diameters Duc and Dc are determined from scans
near the system temperature and scans near 95 ∘C, respectively.
Next, the fitted mode diameters are binned into 3 K intervals
(Marsh et al., 2018) and the mean geometry factor is
derived for each bin. These data are then fitted to a logistic curve,

(2)ξ=1+31+exp-kT-Tr,

where k represents the steepness, T is the measured temperature of the
conditioning loop, and Tr is the relaxation temperature
representative of the midpoint of the logistic curve. The shape factor is
converted to viscosity, η, via a lookup table of modified Frenkel
sintering theory (Pokluda et al., 1997; Rothfuss and Petters, 2016).
Conditioner residence time, monomer diameter, and surface tension assumed in
the conversion are t=5 s, Dmono=50 nm, and σ=0.034 J m−2, respectively. The assumed surface tension is based on
measured values for polyethylene melts (Dettre and Johnson, 1966; Owens and
Wendt, 1969). The inferred viscosity is only weakly dependent on the assumed
surface tension value (Marsh et al., 2018; Rothfuss and Petters, 2016).
Particle shape factors vary between 1≤ξ≤4, which map to a narrow
range of viscosity between 10−5 and 3×10-6 Pa s for the
selected conditioner residence time, monomer diameter, and surface tension.
The inferred temperature-dependent viscosity over this range is fitted to a
modified Vogel–Fulcher–Tammann (VFT) equation (Rothfuss and Petters, 2017).

(3)log10η=A+BT-T0,

where A, B, and T0 are coefficients obtained from a least-square
fit. By convention (Debenedetti and Stillinger, 2001), the glass transition
temperature, Tg, corresponds to η=1012 Pa s. It is
obtained by extrapolation of the VFT equation. Similarly, the temperature of
the transition where η=5×106 Pa s, Tc, is
computed from Eq. (3). This value is computed to aid comparison of transition
temperatures from similar measurements made with this experimental setup.

2.1.2 Pure compound CCN measurement

The CCN instrument was calibrated using size-resolved CCN measurements.
Size-resolved measurements of pure PE were also obtained. Ammonium sulfate
was used to calibrate the CCN instrument supersaturation, which is set by the
streamwise thermal gradient. Gradients between 6 and 20 ∘C were
used. Ammonium sulfate experiments were performed using the system described
in Petters and Petters (2016). Dried charge-neutralized particles are passed
through a DMA (TSI 3080) operated with a sheath-to-sample flow ratio of
9 L min−1: 1.3 L min−1. Particle concentration is measured
using a condensation particle counter (TSI 3771) and the CCN instrument. The
instrument is operated in scanning particle sizer mode with voltage scan
from 10 kV to 10 V. Activation diameter was obtained by a fit to the data
and mapped to supersaturation (SS) using water activity from the Extended
Aerosol Inorganics Model (E-AIM; Clegg et al., 1998). The resulting
relationship between SS and temperature gradient is shown in the Supplement.

The CCN activity of sintered PE was measured using a similar setup. A
high-flow DMA column (Stolzenburg et al., 1998) operated at
9:2 L min−1 flow ratio was used. The high-flow column was used to
access larger-particle diameters. The CCN instrument was operated at a
gradient of 20 ∘C and a total flow rate of 1 L min−1 to
maximize the instrument supersaturation. First, a calibration with AS
particles was attempted but particles activated at all sizes. Another
calibration material, glucose, was used. Glucose was selected because it is
relatively less CCN active than AS (Petters and Petters, 2016; Rosenørn et
al., 2006), forms near-spherical test particles (Suda and Petters, 2013), and
water activity vs. composition is known from bulk data (Miyajima et al.,
1983).

2.1.3 Morphology CCN measurements

Morphology CCN experiments proceeded on dimers with the thermal conditioner
at room temperature (dumbbell morphology) and the thermal conditioner at
∼ 95 ∘C (spherical morphology). Experiments were performed for
AS and PE monomers as well as PE ∕ PE and PE ∕ AS dimers with
different volume fractions. The CCNc was operated ramping nominal temperature
gradients from 7 to 16 ∘C (0.30 % to 0.66 % SS) in discrete
steps. At each step, three SMPS scans were performed. For PE ∕ AS dimers with
different volume fractions (shell thickness), 50 nm diameter AS monomers
were coagulated with either 60, 80, 100, or 120 nm diameter PE monomers.
These dimers were analyzed using the same method but limiting the nominal
temperature gradients between 10 and 15 ∘C (0.42 % to 0.62 %
SS).

The mobility diameter of the dimer depends on the particle morphology. From
previous experiments with similar setups we found that the mobility diameters
of uncoalesced dimers of equal size are between 1.04 and 1.1 times larger
than the mobility diameter of coalesced dimers (Marsh et al., 2018; Rothfuss
and Petters, 2016, 2017). The dry particle masses are identical in the
coalesced and uncoalesced states. Furthermore, as shown below, the PE itself
is CCN inactive and does not contribute to the solute effect. Thus, the
activation of PE ∕ AS dimers is solely controlled by the AS monomer mass,
which is unchanged in all experiments, and the kinetics of water transfer to
the activating drop. The latter factor may differ with morphology and by
extension mobility diameter. Therefore, the activated fraction was determined
as follows. The monomer or dimer size and CCN response functions were fitted
to a lognormal distribution function. The activated fraction (AF), i.e., the
ratio of CCN to CN, was taken at the mode diameter of the size distribution.
The activated fraction was then calculated for each scan at a particular
supersaturation. A Gaussian cumulative distribution function was used
to fit the AF vs. SS relationship. The activation supersaturation SS50
corresponding to the SS at which AF equals to 0.5 was considered to be the
critical supersaturation (Sc; Petters et al., 2009).

3.1 Pure compound CCN experiments

Raw data for the pure compound CCN experiments are provided in the Supplement. For PE monomers no activation was observed at the largest
diameter for which sufficient particle counts could be generated (741 nm)
and thermal gradient in the CCN instrument (20 ∘C). The exact
supersaturation corresponding to the thermal gradient is unknown because most
of the reference glucose particles of the smallest selected particle size
activated at this setting (23.4 nm). Taking glucose κ=0.17 based on
CCN measurements (Petters and Petters, 2016), this implies that
SS >2.5 %. The combination of SS =2.5 % and D=741 nm
corresponds to a state above the Kelvin condition where insoluble but
wettable particles activate (κ=0, Petters and Kreidenweis, 2007).
Polyethylene is hydrophobic and has a contact angle with water between 79 and
111∘ (Boulange-Petermann et al., 2003; Dann, 1970; Gotoh et al.,
2000; Owens and Wendt, 1969). The absence of measurable CCN activity under
the selected conditions is consistent with nucleation theory that includes
contact angle (Fletcher, 1962; Mahata and Alofs, 1975). The results show that
in mixed particles that are composed of AS and PE, the PE will not contribute
dissolved solute. Therefore, a 50 nm dry AS particle and 50 nm dry AS
particle mixed with PE are expected to activate at the same critical
supersaturation provided that the contribution of PE to the wet particle
volume at the point of activation remains small.

Table 1Tabulated values for viscosity experiments. Symbols not defined
above: ΔT is the 95 % confidence interval of Tr
determined from the fit, and RH is the relative humidity inside the
coagulation chamber.

3.2 Viscosity experiments

Two experiments were performed: temperature-induced relaxation of PE ∕ PE
dimers and relaxation of PE ∕ AS dimers. Results from the two
experiments are graphed in Fig. 2. The graphs are included to illustrate the
data reduction procedure through Eqs. (2) and (3). Fitted parameters from
these experiments are summarized in Table 1. The main conclusions from these
experiments are as follows. At T>80∘C, the shape of the
particles no longer changes and the particle is fully coalesced. At those
temperatures, η<105 Pa s. Extrapolation of the VFT curve shows that
at T∼20∘C the estimated viscosity for PE is 1012 Pa s,
which corresponds to a highly viscous, potentially glassy, state. This
extrapolation method has been shown to predict Tg within
±10∘C for sucrose and citric acid (Marsh et al., 2018; Rothfuss
and Petters, 2017). Estimated Tc and Tr values
between the two experiments are within 4 ∘C, consistent with prior
results that show that relaxation of a viscous liquid around a solid particle
produces similar results than the merging of two liquefying spheres (Rothfuss
and Petters, 2017). The precise morphology of the fully coalesced PE ∕ AS
dimer is unknown. The change in mobility diameter for the PE ∕ PE and
PE ∕ AS experiments was 5.96 and 5.88 nm, respectively. For PE ∕ PE
particles, this is consistent with expected dynamic shape factors for rods
and the final diameter of the coalesced particles is spherical. The
similarity of the shift suggests that PE ∕ AS particles are also
spherical, with liquid PE coating the AS at high temperature.

Figure 3CN and CCN size distributions at three different CCNc column
temperature gradients corresponding to water supersaturations measured after
the thermal conditioning of particles.

3.3 Morphology CCN experiments

Figure 3 shows a typical morphology CCN experiment. The curves correspond to
the SMPS scan using the CN and CCN as detector. The Fig. 3a, d,
and g correspond
to measurements with AS monomers. For this experiment, TSI-DMA2 and the
electrostatic filter were turned off. Thus, the experiment corresponds to a
tandem DMA experiment with a second neutralizer placed in line. The
resulting size distribution is trimodal over the range shown. Theoretical
analysis and experimental verification of the size distribution produced by
this tandem DMA configuration is provided elsewhere (Petters, 2018;
Wright et al., 2016)1. Briefly, the central peak is dominated by
singly charged particles transmitted by TSI-DMA1, the peak to the right is
dominated by doubly charged particles that have undergone charge
re-equilibration, and the peak to the left is dominated by singly charged
particles transmitted by TSI-DMA1 that have acquired two charges during the
charge re-equilibration step. At dT=16 K (SS = 0.66 %) the CCN and CN
distributions agree, indicating that particles of all sizes activated.
Conversely, at dT=8 K (SS = 0.34 %) none of the nominal 50 nm AS
monomers activated, while most of the larger particles did. At dT=12 K
(SS = 0.50 %) an appreciable fraction of the 50 nm AS monomers
activated.

Figure 4Activated fraction determined at mode diameter of ammonium sulfate
monomers and mixed ammonium sulfate ∕ polyethylene particles with
dumbbell and core-shell morphologies at different water supersaturation. Lines
correspond to a fit to the data.

Fig. 3b, e, and h correspond to measurements with 50 nm
PE ∕ 50 nm AS dimers in their uncoalesced state (dumbbell morphology).
The measured size dimer distribution is also trimodal, although for reasons
different to those of the tandem DMA experiment. The shape of the dimer
size distribution has been analyzed elsewhere (Petters, 2018; Rothfuss and
Petters, 2016). Briefly, the central peak is dominated by dimers formed from
+1/-1 charged particles transmitted by
TSI-DMA1 and TSI-DMA2. The peak to the right is dominated by dimers formed
from coagulated +2/-2 charged particles.
The peak to the left is dominated by monomers that lost their charge on
transit between either size-selection DMA or the electrostatic filter. Note
that at the mode of the central peak, the solute volume of AS is identical to
that of the monomer experiment shown in the Fig. 3a, d, and g. Consequently, the
central peak is used for analysis. The ratio of CN and CCN at the mode
diameter of the central peak for dT=8, 12, and 16 K are very similar to
that of the monomer experiment. The only difference between the middle panel
and right panel in Fig. 3, i.e., between Fig. 3b, e, and h and Fig. 3c, f, and i, is the temperature of thermal conditioner. The
central mode corresponding to the +1/-1
dimers is reduced by 4.4 nm (Table S8 in the Supplement), indicating the
change in particle morphology while holding the number of AS and PE
molecules comprising the particle constant. The dimers in their coalesced
state represent the core-shell morphology. Assuming a uniform coverage,
the thickness of the PE shell is 6.5 nm. Again, the ratio of CN and CCN at
the mode diameter of the central peak for dT=8, 12, and 16 K are very similar
to that of the monomer experiment.

Figure 4 shows the relationship between AF and SS at the mode diameter for
50 nm AS monomers, uncoalesced 50 nm AS ∕ 50 nm PE dimers, and fully
coalesced 50 nm AS ∕ 50 nm PE dimers shown in Fig. 3. Here the discrete
supersaturation values correspond to steps in dT=1 K, i.e., dT=8, 9,
…, 16 K (0.34 % to 0.66 % SS). The fitted activation spectra
for the three experiments are virtually identical. Although there is some
scatter in the AF data, the activation occurred at the same dT in all three
cases. This implies that the CCN activation is controlled by only the AS
core. Addition of hydrophobic PE to the particle has no effect, irrespective
of particle morphology.

The experiment was repeated with 50 nm AS and 60, 80, 100, and 120 nm size
PE dimers to vary the nominal coating thickness. These experiments are
summarized in Fig. 5. The volume fraction of PE varied from 50 % to
96.7 %, corresponding to nominal coating thicknesses between 6.5 and
36.4 nm. The dimer data were obtained in the coalesced state and the monomer
data are shown for reference. Again, no significant differences in the
activation supersaturation are observed. In some experiments, the activated
fraction did not approach unity at high supersaturation. For experiments with
disparate diameter of AS and PE, e.g., 50 nm AS and 120 nm PE, the dimer
diameter will approach that of the larger monomer. Since a small fraction of
decharged monomers also transmit (Rothfuss and Petters, 2016; Petters,
2018), the distribution contains some fraction of pure PE particles that will
not activate. Thus, the lack of 100 % activation at high SS is likely an
artifact and not due to particle morphology. However, it is also possible
that those particles are composed of PE ∕ AS and have assumed a
core-shell morphology that is sufficient to hinder activation.

A question about how organic coatings influence CCN activity dates back to
Bigg (1986), who attributed poor aerosol-to-CCN closure
to organic coatings. The two principal mechanisms of the delay are poor mass
accommodation and diffusional limitation through the organic matrix (e.g., Chuang et al., 1997; Nenes et al., 2001; Zobrist et al., 2011). Either
effect also implies some influence of particle morphology on CCN activation.
Core-shell structures might be shielded by the organic while agglomerates or
homogeneous mixtures are not.

Hygroscopic organic compounds that dissolve will not shield inorganic
substances, even if they are initially glassy. Dissolution kinetics in
crystalline substances occur on timescales faster than exposure time in CCN
instruments (a few seconds) or cloud updrafts (a few minutes)
(Asa-Awuku and Nenes, 2007). This is corroborated by CCN
experiments with sparingly soluble hygroscopic compounds that activate at
their deliquescence RH, e.g., succinic or adipic acids. In their most pure
state, these compounds activate according to theoretical prediction from
solubility
(Bilde and Svenningsson, 2004; Christensen and Petters, 2012; Hings et al., 2008;
Hori et al., 2003; Kreidenweis et al., 2006), which implies no kinetic
limitation to deliquescence and dissolution for crystalline solids at the
timescale of CCN experiments. Amorphous glassy particles correspond to a
less ordered state than crystalline solids. One might therefore expect a
lower barrier to dissolution. Rothfuss et al. (2018)
measured the condensation kinetics of water on amorphous glassy hygroscopic
organic supermicron particles and found only marginal delays due to high
starting viscosity. Extrapolation to submicron scales yields equilibration
times ∼ 100 ms, too fast to affect CCN experiments. The cited
fundamental arguments are also consistent with the finding that water
soluble and/or hygroscopic organic compounds have not shown growth delays or
activation delays in coating studies (Cruz
and Pandis, 2000; Garland et al., 2005; Hämeri et al., 1992). Based on
this evidence, hygroscopic glassy organic substances can be ruled out to
affect CCN activity through growth delays, irrespective of particle
morphology.

Hydrophobic liquid organic substances may shield the inorganic core through
mass accommodation effects. The studies discussed in the introduction (Abbatt
et al., 2005; Nguyen et al., 2017; Xiong et al., 1998) suggest that thick
coatings of lauric, stearic, or palmitic acids are needed to achieve
noticeable growth delays or deviation from ZSR mixing. The results by Nguyen
et al. (2017) show a remarkable difference between saturated and unsaturated
fatty acids, with saturated fatty acids hindering water transfer. It is known
that the barrier efficiency increases with increasing organic chain length
and packing density (McNeill et al., 2013). Packing density increases with
attractive forces between molecules; the surface-active fatty acids can bind
to the hydrophobic tails and hydrophilic end groups. Lauric (C12) and
palmitic (C16) acids are also macroscopic solids at room temperature due
to their melting points of 44 and 63 ∘C, respectively (Lide and
Haynes, 2009). These values are 63 and 44 ∘C cooler than that of the
PE standard used here (107 ∘C). Assuming a similar shape of the VFT
curve for fatty acids and PE, this implies that the viscosity of amorphous
palmitic acid coatings at room temperature is <106 Pa s. Some
retardation based on viscosity is therefore expected for the fatty acids,
although the viscosity is orders of magnitude lower than that of the glassy
PE. The experiments here were motivated to generate core-shell structures
that are both glassy and hydrophobic to investigate if the combination of the
two effects will modify CCN activity. Before discussing possible reasons why
no decline in CCN activity was observed, the nature of hydrophobic glassy
particles is discussed.

Organic compounds become viscous and glassy due to the presence of functional
groups. Rothfuss and Petters (2017) rank the sensitivity of viscosity to
functional group addition from most to least sensitive as carboxylic acid
(COOH) ≈ hydroxyl (OH) > nitrate (ONO2) > carbonyl
(CO) ≈ ester (COO) > methylene (CH2). With the
exception of nitrate groups, functional groups that are responsible for high
viscosity also promote hygroscopicity (Petters et al., 2016; Suda et al.,
2014). To our knowledge, multifunctional nitrated organic compounds are not
prevalent or rare in the atmosphere. Therefore, the only source of
hydrophobic glassy particles are high-molecular-weight weakly functionalized
hydrocarbons. The PE source used here had a nominal molecular weight of
850 Da, corresponding to ∼ 60 CH2 groups. The carbon number
may have changed during heating via thermal decomposition. Whether or not
this occurred cannot be determined from the available data. Regardless, the
obtained PE particles had an estimated viscosity of 1012 Pa s at room
temperature. The exact carbon number of the PE particles here is not
important, addition or subtraction of a few CH2 groups does not
significantly alter viscosity or hygroscopicity (Petters et al., 2016; Rothfuss
and Petters, 2017). Note, however, that atmospherically relevant hydrophobic
organics may have double bonds, include aromatic rings (e.g., polycyclic
aromatic hydrocarbons), and may include some functional groups other than
CHx. Nonetheless, the PE particles used here are used as a
proxy for hydrophobic and glassy compounds. What then are plausible reasons
that the PE model did not affect CCN activity?

The experiments in Fig. 5 can be used to bound the water diffusivity. A
nominal coating thickness of 36.4 nm corresponds to a particle composed of
96.7 % organic material by volume. Scaling analysis implies that the
apparent diffusivity of water through the shell is D≥10-16 m2 s−1, much larger than values calculated from the
Stokes–Einstein relation. This estimate assumes a uniform coating. In
practice, the uniformity of the coating is unknown. At 95 ∘C, the
estimated viscosity of PE is <105 Pa s (Fig. 2). The resulting
particles are spherical. If we imagine the system as effectively liquid, the
surface tension forces will lead to a completely engulfed core with uniform
coating. After exiting the conditioning loop, the coating anneals. It is
plausible that small fissures form in the curved shell, which would reduce
the packing density and allow water to penetrate. It is also plausible that
the resulting coating is not of uniform thickness due to annealing forces
resulting in the core to be pushed off center. In the extreme case a
partially engulfed morphology may result, which has been documented to occur
in particles with slow drying rates (Altaf and Freedman, 2017; Nandy and
Dutcher, 2018). Another possibility is that the known breakdown of the
Stokes–Einstein equation (Chenyakin et al., 2017; Price et al., 2014) is
strong enough to result in diffusion rates through glassy PE that exceed the
apparent D≥10-16 m2 s−1. Indeed the measured
diffusivity of water through low-density PE films is ∼10-14 m2 s−1 (Wang et al., 2011). Thus, any of the mentioned
effects (cracks, nonuniform coating thickness, or faster than expected
diffusion) could have contributed to, or fully explain, the lack of shielding
observed in this study. Note that the saturated fatty acids for which delays
have been observed have viscosities <106 Pa s, which is low enough to
allow viscous flow at the scale of submicron particles, and consequently the
formation of a tightly packed viscous shell without cracks. This might
suggest that the optimal barrier for shielding particles from water would be
for substances that have a narrow range of viscosity where the coating is
viscous enough to slow water transport but not too viscous to prevent flow
around the particle.

These findings have important atmospheric implications. The permeability of
polymers to small molecules is well known. For example, Nafion membrane
humidifiers are widely used in the community for drying or humidifying
aerosol flows (and are permeable to water vapor). Permeability depends on the
polymer composition, its molecular weight, temperature, and the number and
type of cross-links (George and Thomas, 2001). A key question is what types of
oligomeric or polymeric substances could form atmospheric hydrophobic glassy
coatings, and by what process. Three are imagined here. Long-chain fatty acid
coatings may form on sea-spray particles during the bubble-bursting process
(Tervahattu, 2002). Some of these will initially be liquid and might turn
glassy upon cooling or functionalization via heterogeneous aging. Weakly
oxidized oils in the C18-C40 range are emitted by diesel
engines (Sakurai et al., 2003). These compounds may form coatings on
pre-existing atmospheric particles perhaps through an
evaporation–oxidation–condensation mechanism (Robinson et al., 2007) and also
become glassy upon cooling or functionalization via heterogeneous aging. A
third possibility is the irreversible oligomerization of weakly
functionalized low-molecular-weight organic compounds forming hydrophobic
compounds that then deposit on the surface of inorganic particles during
drying (Altaf and Freedman, 2017; Nandy and Dutcher, 2018), followed by
turning glassy during cooling. It is unclear how common and important these
processes might be in the atmosphere. To our knowledge single particle
studies on dried aerosol do not show core-shell structures as the dominant
particle type (Laskin et al., 2012; Li et al., 2010; Piens et al., 2016).
Regardless, the imagined processes producing hydrophobic glassy coatings
undergo drying and cooling cycles that will be susceptible to the same issues
reported here: cracks, nonuniform coating thickness formed during drying or
annealing, partially engulfed equilibrium morphology, and faster than
expected diffusion through hydrocarbon films. We therefore conclude that mass
transfer limitation by glassy organic shells is unlikely to affect cloud
droplet activation in the majority of cases at temperatures prevalent in the
lower atmosphere. Extension of this result to temperatures in the upper free
troposphere where low temperatures slow diffusion may require further
experimentation.

Altaf et al. (2018) report experiments that suggest that dry particle
morphology impacts the activation diameter. These experiments were performed
with particle mixtures composed of 50∕50w∕w mixtures of
AS ∕ succinic and AS ∕ pimelic acid particles. Particle morphology
was varied by changing the drying rate; a slow drying rate produces particles
with partially engulfed morphologies while a fast drying rate produces
particles with homogeneous morphology. The authors report that the partially
engulfed morphologies activate at a smaller diameter than the homogeneous
morphology. This result is opposite to the one reported here, where no
effect of particle morphology on CCN activity was observed. The experiments
here and by Altaf et al. (2018) differ in two key aspects. First, organic
material between the two studies is different. Polyethylene is hydrophobic
and does not contribute to the solute effect. Succinic and pimelic acids do.
Succinic and pimelic acids also slightly lower surface tension of the
air–aqueous solution interface, while PE does not. Second, the amount
of solute is controlled differently in this study. Here, dimer morphology is
produced by coagulation with fixed AS monomer size. In contrast, Altaf et
al. (2018) used atomized mixtures, followed by drying and size selection.
Altaf et al. (2018) do not have a quantitative theory to predict the observed
effect of particle morphology but they propose potential explanations. One
proposed explanation is that pimelic acid serves as a heterogeneous catalyst; the
other proposed explanation is a liquid–liquid phase separation mechanism
coupled with surface tension lowering similar to the mechanism proposed by
Ovadnevaite et al. (2017). Neither of these effects is applicable to the PE
model system. However, we point out that the methodology used here can be
used to create core-shell structures with many combinations of inorganic
cores and organic coatings to further investigate the effect of particle
morphology on CCN activation for core-shell structures with organic shells
that may lower surface tension.

Dimers composed of ammonium sulfate (AS) and polyethylene (PE), and PE and PE,
were prepared. The dimers were used to determine the temperature dependence
of the viscosity of the generated PE particles by measuring the conditions
when dimers coalesce and relax to spheres. The viscosity of PE was
5×106 Pa s at 60 ∘C. Extrapolation of the temperature
dependence suggests that the PE particles have a viscosity of 1012 Pa s
near room temperature. Seven hundred nanometer diameter PE particles were CCN
inactive at 2.5 % supersaturation consistent with a contact angle of PE
with water exceeding 79∘. The AS ∕ PE dimers were used to measure
the critical supersaturation of the particles in dumbbell morphology that were formed
during coagulation and core-shell morphology formed after shape relaxation.
No difference in activation supersaturation was observed up to a nominal shell
thickness of 36.4 nm. An increase in supersaturation is expected if water
diffusion was governed by the Stokes–Einstein relation. The apparent water
diffusion through the hydrophobic plastic is orders of magnitude faster than
predicted from the Stokes–Einstein relation. Potential explanations are
cracks formed during annealing, nonuniform coating thickness, formation of
partially engulfed morphologies, or fast diffusion of small molecules through
polymer membranes. It is argued that processes that may form glassy
hydrophobic organic shells on atmospheric particles will result in similar
imperfect shielding of hygroscopic cores. However, particles with thick
coatings of some, but not all, fatty acids are an exception to the preceding
claim (Abbatt et al., 2005; Nguyen et al., 2017; Forestieri et al., 2018).
Water transfer will be less hindered in hydrophilic glassy organic materials
due to the plasticizing effect of water on dissolving organic compounds.
Based on literature data, particles comprising hydrophobic glassy organic
materials fully coating inorganic cores are not ubiquitous in the atmosphere.
Furthermore, timescales of humidification are shorter in CCN instruments than
in atmospheric updrafts. Therefore, our experiments suggest that, near
laboratory temperatures, mass transfer limitation by glassy organic shells is
unlikely to affect cloud droplet activation.

This research was funded via the Department of Energy, Office of Science grant
DE-SC 0012043. Ankit Tandon acknowledges the Central University of Himachal
Pradesh for granting study leave to carry out this work at the North Carolina
State University.

Theoretical analysis of the relevant response functions reported in this work is
provided in Notebook S8. Hygroscopicity Tandem DMA and
Notebook S10. Dimer Coagulation and Isolation, which are
supplements to Petters (2018).

Organic compounds may form a barrier to condensation. Such barriers have been hypothesized to prevent water and other substances from mixing with salt cores. This will hinder the particles' ability to aid cloud formation of

Organic compounds may form a barrier to condensation. Such barriers have been hypothesized to...